Changes in the functional properties of ACh receptor channels occur during development of amphibian and mammalian skeletal muscle. The proposal addresses two central questions relating to regulation of changes in receptor function. First, what molecular mechanisms are involved in the change in ACH receptor channel function? Second, can it be demonstrated that nerve plays a role in causing the change in function? The experimental approach is to study both the developmental changes in receptor function on embryonic Xenopus muscle and the change in receptor function in response to denervation of mature Xenopus tadpole muscle. Advantage is taken of the fact that both embryonic and mature Xenopus muscle can be grown in tissue culture. The developmental change in receptor kinetics observed in vivo, continues to occur on tissue-cultured Xenopus muscle. Changes in receptor kinetics also occur in tissue-cultured mature Xenopus muscle in response to denervation. Time-dependent changes in receptor kinetics will be examined in both developing and mature Xenopus muscle by use of combined single channel recording and focal noise technique. Nerve independent changes in kinetics will be studied in tissue cultured muscle grown in the absence of nerve. Nerve dependent changes in kinetics will be examined by functional reinnervation of muscle in culture. The kinetic differences between synaptic and non-synaptic receptors will be studied by use of biochemical labelling techniques which identify synapses on cultured muscle. Finally, the advantages of tissue culture will be utilized to test the hypothesis that changes in receptor kinetics results form a modification of pre-existing receptors. These proposal experiments form the basis for establishing a system by which the molecular mechanims underlying change in functional properties of receptors can be elucidated.